Dotted around Alola are various “boss” trainers, who you can only fight after you’ve defeated all the other trainers in the route. Taking it one step further, there’s a trainer near the Mount Lanakila Pokémon Center, who you can only fight after defeating all the boss trainers. So effectively the… boss, boss trainer?
Anyhow, if you’re missing some trainers to defeat, just ask him for their general location.
All of Aristo’s Pokémon share something in common: a held item that prolongs battle effects. His first one, Carbink, has a Light Clay, which extends the duration of its Reflect and Light Screen. Besides those two moves, it can use Stealth Rock to cause damage when you switch in, but no attacking moves. Smash it with Steel-types ASAP.
Torkoal has a Heat Rock that lengthens the duration of sunny weather, which it can trigger with Drought when it’s sent into battle. For attacks, it can use Fire Blast, which receives a power boost from the sun, and Solar Beam to hurt Water, Ground and Rock-types (skipping the charging turn during the sun). If possible, cancel its weather with your own or Cloud Nine etc.
Likewise, Pelipper has a Damp Rock that increases the duration of rain, which it will trigger with Drizzle as soon as it enters battle. It can use Hydro Pump, which is boosted thanks to the rain, and Hurricane, which has 100% accuracy in the rain. Electric-types are its bane, especially since Pelipper isn’t known for being fast, like most Electric-types.
Meanwhile his Alolan Ninetales has an Icy Rock that extends the duration of hail, triggered by its Snow Warning Ability. Attack-wise, it can use Blizzard, which has perfect accuracy when there’s hail, and Dazzling Gleam. Thankfully, hail is much less annoying than the sun or rain. Anyway, Steel-types are ideal; Fire, Rock and Poison-types are also good.
Finally, Gigalith has a Smooth Rock that makes sandstorms last longer; of course, a sandstorm will be conjured by its Sand Stream Ability. For attacks, it has Stone Edge for STAB, and Earthquake for hitting Steel-types. Like hail, sandstorm isn’t too bad, but keep in mind it will boost Gigalith’s Special Defense. Water or Grass-types are recommended.
After winning, you’ll receive a Life Orb, which increases the Pokémon’s damage output at the cost of inflicting a small amount of damage to itself. For best results, give it to a Pokémon with exceptional Attack or Special Attack. Or a Pokémon with the Unaware ability, which will passively negate the recoil damage.